1. Field of the Invention
The present invention relates to an information signal recording apparatus, an information signal recording method, an information signal reproducing apparatus, and an information signal reproducing method.
2. Description of the Related Art
Recording and reproducing apparatuses corresponding to the digital video (DV) format for recording and reproducing a digital video signal, for example a camcorder (which is a general product name derived from camera and recorder), have been widespread. The following Patent Document 1 describes an apparatus that records and reproduces, by using a tape and a rotating head unit that are the same as those of a DV format camcorder, high definition (HD) data that have been compressed in accordance with MP@H-14 prescribed in the Moving Picture Experts Group Phase 2 (MPEG2) system. The contents described in Patent Document 1 are referred to as the HDV2 standard.
[Patent Document 1] Japanese Patent Laid-Open Publication No. 2002-314941
The patent document 1 describes a technology for forming search picture data with I pictures, causing intervals of intraframes used as search data constant, and decreasing the data amount of high speed search pictures. The MPEG2 system uses intra-coded pictures (I pictures), predictive-coded pictures (P pictures), and bidirectionally predictive-coded pictures (B pictures) in accordance with different encoding systems.
An I picture is a picture of which, when it is encoded, only information thereof is used. Thus, an I picture can be decoded without need to use other information. When a search operation is performed, since the path of a rotating head does not match a track formed on a tape, only a fragment of data is reproduced. Thus, it is necessary to form search pictures with I pictures.
A P picture is a picture that uses an I picture or a P picture that has been decoded chronologically before the current P picture as a predictive picture (a reference picture for obtaining the difference between two pictures). The difference between the current P picture and a predictive picture that has been motion-compensated is encoded. Alternatively, the current P picture is encoded. One of the two methods is selected in the unit of a macro block so that a higher effect can be obtained.
A B picture is a picture that uses three types of pictures that are an I picture or a P picture that has been decoded chronologically before the current B picture, an I picture or a P picture that has been decoded chronologically after the current B picture, and an interpolated picture formed with the foregoing two types of pictures, as a predictive picture (a reference picture for obtaining the difference between two pictures). Based on the three types of reference pictures that have been motion-compensated the differences are encoded. Alternatively, the three types of pictures are intra-encoded. One of these methods is selected in the unit of a macro block so that a higher effect can be obtained.
Thus, there are four types of macro blocks that are an intra-frame coded macro block of which a current macro block is encoded in a frame, a forward inter-frame macro block of which a future macro block is predicted with a past macro block, a backward inter-frame predictive macro block of which a past macro block is predicted with a future macro block, and a bidirectional macro block of which a current macro block is predicted from both the forward and backward directions. A P picture contains an intra-frame coded macro block and a forward inter-frame predictive macro block. A B picture contains all the four types of macro blocks.
In Patent Document 1, to decrease the data amount of search pictures, DC components are extracted from discrete cosine transform (DCT) blocks of a luminance signal and color difference signals and the number of bits of each of the extracted DC components is decreased from the original 8 bits to 6 bits (for each of DC components of the luminance signal) and 5 bits (for each of DC components of the color difference signals).
Since the number of bits that are decreased is large, from 8 bits to 5 bits, the possibility that small color difference data is truncated to 0 is high. Video data photographed by for example a video camera contains many portions that have low signal levels. Depending on a picture pattern, most of a picture on a screen may be truncated to 0. As a result, search pictures may be formed as colorless pictures. Search pictures can be coarse pictures. However, if search pictures are colorless, the user may feel uncomfortable or mistake them for a trouble of the apparatus.